A 21 blood glucose is extremely low, so one would expect
signs in this ferret. As Dr Williams mentioned "lab error" is
a possible explanation. Did the clinic run the test there at
the clinic and give you the results in a couple of minutes or
did they send the bloodwork out to a lab? It would be a good
thing to do just a glucose there at the clinic (on a glucose
machine for diabetics). That way you know if it is really that
low or just an error.
Jerry Murray, DVM